1. Technical Field
The present invention relates to piston rings which have good nitridability. Furthermore, the present invention relates to a nitrided piston ring which can be manufactured from the piston ring with good nitridability of the invention. In addition, the present invention relates to a process for the manufacture of the piston rings with good nitridability of the invention and to a process for the manufacture of the nitrided piston rings in accordance with the invention.
2. Related Art
In an internal combustion engine, piston rings seal the gap between the piston head and the cylinder wall of the combustion chamber. As the piston moves back and forth, one side of the piston ring slides with its outer circumferential surface against the cylinder wall in a permanently spring-loaded position, and because of the tilting movements of the piston, the other side of the piston ring slides in an oscillating manner in its piston ring groove, whereupon its flanks bear alternately on upper or lower groove flanks of the piston ring groove. The mutual sliding of these components against each other results in a greater or lesser amount of wear, depending on the material; if it runs dry, this can lead to so-called fretting, scoring and finally destruction of the engine. In order to improve the slide and wear behaviour of the piston rings against the cylinder wall, their circumferential surface has been provided with coatings formed from various materials.
In order to produce high performance internal combustion engine parts, such as piston rings, cast iron materials or cast iron alloys are usually used. In high performance engines, the requirements placed upon piston rings, in particular compression rings, are becoming ever more stringent, for example as regards peak compressive pressure, combustion temperature, EGR and lubricant film reduction, which substantially affect their functional properties such as wear, scorch resistance, micro-welding and corrosion resistance.
Prior art cast iron materials, however, are at great risk of breaking; in fact, when using current materials, the rings frequently break. Increased mechanico-dynamic loads result in shorter service lifetimes for piston rings. Severe wear and corrosion occurs on the running faces and flanks.
Higher ignition pressures, reduced emissions and direct fuel injection mean increased loads on the piston rings. This results in damage and a build-up of piston material, especially on the lower piston ring flank.
Because of the higher mechanical and dynamic stresses on piston rings, more and more engine manufacturers are demanding piston rings from high-grade steel (hardened and tempered and high alloy, such as grade 1.4112, for example). Ferrous materials containing less than 2.08 weight % of carbon are herein known as steel. If the carbon content is higher, it is known as cast iron. Compared with cast iron, steels have better strength and toughness properties as there is no interference from free graphite in the basic microstructure.
Usually, high chrome alloyed martensitic steels are used for the manufacture of steel piston rings. However, using such steels suffers from the disadvantage that the manufacturing costs are significantly higher than those of cast iron components.
Steel piston rings are manufactured from profiled wire. The profiled wire is coiled into a circular shape, cut and pulled over a “non-round” mandrel. The piston ring attains its desired non-round shape on this mandrel by means of an annealing process, which imparts the required tangential forces. A further disadvantage of the manufacture of piston rings from steel is that beyond a certain diameter, ring manufacture (coiling) from steel wire is no longer possible. Piston rings formed from cast iron, on the other hand, are already non-round when cast, so that from the outset they have an ideal shape.
Cast iron has a substantially lower melting point than steel. The difference may be up to 350° C., depending on the chemical composition. Thus, cast iron is easier to melt and to cast, since a lower melting point means the casting temperature is lower and thus the shrinkage on cooling is smaller, and so the cast material has fewer pipe defects or heat and cold cracking. A lower casting temperature also results in a lower stress on the material of the mould (erosion, gas porosity, sand inclusions) and the furnace and also results in lower melting costs.
The melting point of a ferrous material does not simply depend on the carbon content, but also on its “degree of saturation”. The following empirical formula applies:Sc=C/(4.26−1/3(Si+P))
The closer the degree of saturation is to 1, the lower is the melting point. For cast iron, a degree of saturation of 1.0 is usually desirable, whereupon the cast iron has a melting point of 1150° C. The degree of saturation of steel is approximately 0.18, depending on the chemical composition. Eutectic steel has a melting point of 1500° C.
The degree of saturation can be substantially influenced by the Si or P content. As an example, a 3 weight % higher silicon content has a similar effect to a 1 weight % higher C content. Thus, it is possible to manufacture a steel with a C content of 1 weight % and 9.78 weight % silicon content which has the same melting point as cast iron with a degree of saturation of 1.0 (C: 3.26 weight %; Si: 3.0 weight %).
A drastic increase in the Si content can raise the degree of saturation of the steel and reduce the melting point to that for cast iron. Thus, it is possible to manufacture steel with the aid of the same technology that is used for the manufacture of cast iron, for example GOE 44.
Piston rings formed from high silicon cast steel are known in the art. However, the silicon present in larger quantities has a negative influence on the hardenability of the material since its austenite transition temperature, “Ac3”, is increased.
A process that is normal in the art for nevertheless increasing the hardness of the piston ring surface could consist in nitriding the material. However, it has been shown that prior art high silicon steel castings have poor nitridability.